That's about after-images and what your brain interprets. When the mouse cursor is moved over the image, it switches to black-and-white, which supplies your brain with textural info. The after-image effect is the chemicals present on your retina, which on a plain white background would be a blurry image at best, but because it is super-imposed over the b/w image, your brain mixes the b/w image with the after-image, and it results in the original color image.
Peace, Love, and Vegetable Rights!
Eat Meat and Save the Plants!